Although there have been many schemes for varying the torque bias ratio between drive axles connected by a differential, all of them have experienced problems; and the open differential is still the most popular one in use. This has a relatively efficient torque transmission train and a low bias ratio, which works well so long as both wheels have traction. The low bias ratio makes it easy to get stuck if one wheel loses traction, however, because not much torque can be transmitted to the opposite wheel.
The many attempts to overcome this have mostly involved reducing the efficiency of the interaxle torque transmission through the differential. This increases the bias ratio and increases the torque that can be transmitted to one wheel when another one slips. A high and constant bias ratio can cause other problems, however. Since nearly all differentials have an interaxle speed ratio of -1 (meaning that axles rotating relative to each other do so in an opposite direction at the same speed), and since all differentials have efficiencies of less than 100%, those with a -1 speed ratio always apply a proportionally larger amount of torque to the slower rotating axle on the inside of a curve. Differentials with a high bias ratio exaggerate the greater proportion of torque applied to the inside wheel and thus create an under steer moment, urging the vehicle straight ahead while it is turning.
Many proposals have also been made for differentials having interaxle drive efficiencies that can be varied during operation. Fluid couplings and friction clutches are among the possibilities. Although achieving some success in spite of complexity and reliability problems, none of these (nor any other differential with a -1 speed ratio) can apply a proportionally larger amount of torque to the faster rotating axle on the outside of a curve. This can be desirable because the vehicle weight shifts to the outside wheel on a curve, and the more heavily weighted wheel has more traction with which to exert the available torque.
It has also been proposed in U.S. Pat. No. 4,535,651 (CHAMBERS) to vary speed ratios of a differential by means of multiple gear trains that can be engaged alternatively to increase the torque to a slower moving axle if necessary. This may help tractors and slow speed vehicles get a better traction grip, but it would make a high speed automobile unstable to abruptly change its axle speed ratios. Also, shifting gear trains is cumbersome and expensive and, as proposed by Chambers, is still not capable of providing more torque to a faster rotating axle traveling the outside of a curve.
Also proposed in DE, C, 222,138 (LUDWIG MARIA DIETERICH) is a continuously variable transmission which is mounted in a vehicle drive line between an input shaft and two rear drive axles. The transmission provides for controlling speed ratios between the input shaft and each of the drive axles. This effects a steering drive of the type more common in bulldozers and tanks. However, in doing so, the usual function of a differential to permit the output shafts to freely "differentiate" (i.e., rotate at any speed difference between the two which may be required to accommodate a variety of operating conditions including different size drive wheels or uneven distances of travel between drive axles) is lost. Although Dieterich also proposes to control this steering effect by special linkages to a front steering axle, many more variations in addition to the front steering axle position affect the required rotational speeds of the rear drive axles, and any resistance forces active at the rear drive wheels which tend to rotate the drive axles at a speed difference other than the exact difference being controlled would produce undesirable torque "windup" between the axles.
I have discovered a way of transmitting drive torque through a differential to a pair of drive axles in a freely variable manner that allows more of the torque to be sent to the faster rotating axle on the outside of a curve and generally allows the torque distribution to be controlled in response to vehicle driving conditions. My differential is thus able to achieve torque bias ratios not attainable with previous differentials that use torque transmission efficiencies to vary bias ratios. In creating new torque distribution possibilities, my differential allows vehicle suspension and steering design to take advantage of torque variation under different driving conditions such as turning, braking, varying weight distributions, and varying traction. Besides being usable between a pair of opposed axles, my differential can also distribute driving torque between front and rear axle pairs.